The age of apathy stops here, between a Thomas Cook branch and the Bloomsbury Diner, where the bodies are jammed together too tightly to move. In the minutes before the march begins, anyone will tell you why protest has supplanted politics.

Some of these twenty-first century Chartists with mobile phones are veterans of the Vietnam demonstrations. Some are too young to remember the Cold War. What unites them is anger against Bush and Blair, but mainly Blair. Everyone I talk to says that he will not have their vote again.

It is odd to think that these are the sloths who could not be prised from their armchairs when elections rolled round and who hit the remote at the first flicker of any BBC political coverage that wasn't Have I Got News For You.

These people, in New Labour's analysis, were the inert of the Earth. And here they are, out in their hundreds of thousands, quoting Hans Blix verbatim and defying a Prime Minister who longed to galvanise them and must now regret becoming the Frankenstein of the protesting classes.

Political leaders hate crowds. Mass meetings have been supplanted by leaks and soundbites. In the fractious build-up to war, lonely societies are encouraged to become more solipsistic. A fearful population, hiding behind its anthrax-proofed windows, is also tractable. There is nothing threatening to government about citizens bickering over the last roll of duct tape in Wal-Mart.

British marchers have spurned isolation for solidarity, and fear for fury. Their momentum came almost from nowhere. Unlike the Jubilee-trippers, the Soham mobsters and even the Countryside Alliance, they bore no social or political barcode.

Theirs was, and is, a movement without a leader. Its members belong to no obvious political caste. Labour voters who march are deracinated from their leaders, and the Tories have none worth worrying about.

Their mission, to halt the war, is by definition negative, and their goal unattainable, bar a miracle. Those hoping to recalibrate the Prime Minister's moral compass face disappointment, or even despair. Few predicted weeks ago that so many people would turn out to stop the unstoppable, and I was certainly not among them.

The surprise has been the altruism of the protesters, and the size of the vacuum they fill. Blair's natural supporters and opponents have registered their opposition, and seen it spurned. As they get more strident, he digs harder. The hole in democracy grows more cavernous by the day.

The marchers all felt that; the men in deerstalkers and Barbours, the pro-protesters in neon knitwear, the students and the grandmas whose families had persuaded them to take along brand-new Nokia mobile phones they couldn't work. Who will record their assembly when all of this is over?

History, perhaps. The War Remnants Museum in Saigon displays yellowed newspaper reports of Western protests against the Vietnam War. Next door, housed in hot sheds, are the napalmed babies and photographs of burnt children. The juxtaposition of press clippings and grim artefacts offers a memento mori. This is what happens when people are right and governments mistaken.

Today's protesters are starved of inspiration and data. In place of a charismatic leader, they have the belief that politicians are lying. They have no great freedom fighter to support; only Saddam. You could not sell washing powder on that basis, let alone a pacifist cause that may crush a Prime Minister.

Yet the movement has taken off and its subscribers, on yesterday's evidence, are not a reissued set of hoary peaceniks. These are organised people with clear aims. They want a peaceful solution for Iraq. If that is not forthcoming, Blair will be punished accordingly.

They may be wrong. He may be right. But in a war predicated on conviction and conscience, the hunches of the nation also count. As Martin Luther King said, countries should repent citizens' evil deeds almost less than 'the appalling silence of the good'. The unheard have spoken out.